Jökull - 01.01.2010, Side 26

Brandsdóttir et al.
The primary aim of this project was to delineate
the complicated fault network based on detailed af-
tershock locations and determine focal mechanisms
of the bigger aftershocks in order to constrain the
rupture geometry along both faults within the con-
text of the SISZ. An automated picking program was
used for earthquake locations which were compared
with the Icelandic Meteorological Office (IMO) SIL
catalog locations. We used data from the eleven tem-
porary stations (Table 1) and three SIL stations, BJA,
KRO and SOL. The online SIL catalogue was used to
determine event magnitudes (Figure 2) as the CMM
program does not calculate magnitudes. The SIL pro-
vides local moment magnitudes, Mlw (Jakobsdóttir,
2008). Earthquakes larger than 4 may be severely
underestimated by the Mlw scale.
About a week after being deployed station HOLL
was run over by a bulldozer working on a neighbour-
ing road. This resulted in a data gap for about 24
hours until station INGO was rebuilt nearby. The
broadband station UGAR, deployed June 21st was
omitted due to a faulty sensor.
Coalescence Microseismic Mapping (CMM)
The CMM technique of Drew (2010) is an auto-
mated earthquake location process. CMM is a nu-
merical imaging technique while at the same time an
event location algorithm with a theoretical basis in
the Bayesian inversion of arrival times (Tarantola and
Valette, 1982). Rather than reducing the data to dis-
crete arrival time picks, the CMM method continu-
ously maps a short/long time (STA/LTA) average sig-
nal for each phase for each station. The resultant
spatio-temporal maps identify likely times and loca-
tions of events, as points of coalescence of the mapped
signals (Figures 3 and 4). In as far as the STA/LTA sig-
nal relates to the statistical variability of time picks,
where the time picks are taken as the local maxima
of the STA/LTA function, the objective function is the
Figure 2. Local moment magnitudes Mlw of SIL located events May 29th to July 1st, Julian days 150 to 183. A
total of 5737 events. The mainshock moment magnitude Mw 6.3 derived from teleseismic data (CMT-Harvard)
is underestimated by the local network. Grey bars mark deployment of temporary stations. Manual review
of the automatic locations is incomplete for May 30th and June 2nd. – Stærðardreifing eftirskjálfta. Gögn úr
SIL-gagnagrunni Veðurstofu Íslands. Hluti gagnanna er óunninn. Stærsti skjálftinn er vanmetinn á nálægari
stöðvum, hann mældist Mw 6.3 stig á erlendum mælistöðvum. Gráar línur tákna uppsetningartíma færanlegra
jarðskjálftamæla, sbr. töflu 1.
26 JÖKULL No. 60